This invention relates to an x-ray tube for producing a high-efficiency beam, and especially a pencil beam, which is applicable to the field of radiology.
An x-ray tube usually contains an anode and an electron-emitting cathode. The electrons bombard a part of the anode which is known as the anode target and the impact of these electrons on the surface of the anode target produces a focal spot from which x-rays are emitted in all directions. If this x-radiation is limited to a given direction by means of systems for collimation either partly within the interior of the tube or outside this latter, a useful x-ray beam is thus produced and is smaller in all cases than the entire x-radiation emitted at the focal spot. Thus in the case of a radiodiagnostic tube, for example, the useful beam of x-radiation represents approximately 5% of the x-radiation emitted from the focal spot and, in the case of x-ray tubes for tomodensitometry, this percentage is of the order of 1%.
In consequence, in the case of a useful beam having a given intensity which is necessary for a given type of inspection, the electric power supplied to the tube as well as the heat loss will increase in proportion to the degree of collimation produced in order to provide the useful beam with the geometry required by the inspection operation.
By making use of an x-ray tube of conventional design, a useful beam in the form of a pencil beam can be obtained by means of strong collimation. In this case, the useful beam represents a negligible fraction of the x-radiation emitted from the focal spot in respect of a considerable amount of power consumed for the supply of said x-ray tube.